In recent years, demand has been increasing for a high-performance lithium secondary battery to be used for a personal digital assistant, a portable electronic device, a domestic small electric energy storage device, a two-wheeled motor vehicle using a motor as a power source, an electric automobile, a hybrid electric automobile, or the like.
In this case, the secondary battery refers to a battery which can be charged and discharged.
As the purpose of use varies, further improvement in safety of the secondary battery and higher performance thereof have been increasingly demanded.
An inorganic solid electrolyte is inflammable by nature and is a highly safe material compared with an organic electrolyte generally used.
However, the inorganic solid electrolyte exhibits slightly degraded electrochemical performance in comparison with that of the organic electrolyte, and thus, the performance of the inorganic solid electrolyte must be further improved.
Conventionally, an electrolyte exhibiting high lithium ion conductivity at room temperature has been substantially limited to an organic electrolyte.
However, a conventional organic electrolyte contains an organic solvent and thus is combustible.
Thus, use of an ion conductive material containing an organic solvent for an electrolyte of a battery may cause leak or firing.
The organic electrolyte is liquid. Thus, not only lithium ions are conducted but also counter anions are conducted. A lithium ion transport number is 1 or less.
Various studies on a sulfur-based solid electrolyte have been conducted conventionally for solving such problems.
For example, in 1980s, there is reported as a lithium ion-conductive solid electrolyte having high ion conductivity sulfide glass having an ion conductivity of 10−3 S/cm such as LiI—Li2S—P2S5, LiI—Li2S—B2S3, or LiI—Li2S—SiS2.
However, such a solid electrolyte is doped with lithium iodide (LiI) for improving the ion conductivity, and is easily subjected to electrochemical oxidation. Thus, production of an all-solid lithium battery operating at 3 V or more involves difficulties.
Further, pentaphosphorus disulfide (P2S5) to be used as a raw material for the solid electrolyte has problems of toxicity and its industrial use involves difficulties.